Meetup: Redesign
Meetup had been wanting to redesign its web site for years, but it took a radical change in the company structure - an experiment with self-organized teams - for it to get done. Given complete freedom to choose projects, myself and a few great developers decided to ambitiously redesign the site - calling the project "Simplicity" placed the emphasis on streamlining not just the design but the IA and the code behind it as well, all of which had become garbled and cluttered over several years.
The cleaner design led the way to more easily add new features, especially giving groups the ability to customize their themes.